A pseudonoise (PN) sequence has been defined as a binary sequence with a very desirable transorthogonal auto-correlation property, commonly used in space communications for synchronization and ranging (G. P. Kurpis, Chair, IEEE Std 100-1992 The New IEEE Standard Dictionary of Electrical and Electronics Terms, 5th ed. (IEEE, New York, 1993) p. 1024). A direct sequence spread spectrum communication system uses such PN codes. In one such system, a low frequency signal and a high frequency signal are received and amplified. If the low frequency signal occupies a frequency band for example one kilohertz wide centered at 5 megahertz, and the high frequency signal occupies a band for example 10 kilohertz wide centered at 20 megahertz, then if those two signals are run through a common amplifier circuit in separate bands, then the high frequency signal, being in a wider band, will absorb most of the power provided by the amplifier. If the two signals use different frequencies, and their bands do not overlap, then the wider band will absorb most of the available power. As a result, the effective transmission distance for the low frequency signal will be considerably less than that of the high frequency signal. To avoid that problem, the low frequency and high frequency signals can be separately modulated using spreading or PN codes to spread each band to have an identical bandwidth of, for this example, 100 kilohertz each. As so modulated, both the high frequency signal and the low frequency signal now each occupy an equally wide band and absorb power equally, and therefore can be transmitted for essentially the same distance. Previously, this problem would be avoided by using separate power amplifiers for each band so that each band shares power equally. However, such an arrangement has the disadvantage of requiring duplicate (or triplicate or more) circuitry.
In the prior art, it was known that two or more PN component codes could be combined to create a longer and more complex composite code. U.S. Pat. No. 4,809,295 issued Feb. 28, 1989 for "Code Lengthening System" by John W. Zscheile, Jr. et al describes a method and apparatus for generating a composite code having correlation properties between the component codes. Having correlation properties between the component codes is desirable so that a receiver can lock onto a longer such PN code as easily as for a shorter PN code.
In U.S. Pat. No. 4,225,935 issued Sep. 30, 1980 for "Coding Method and System with Enhanced Security" by John W. Zscheile, Jr. et al, the system referenced therein combines individual PN component codes to provide a PN composite code having a code length equal to the product of the individual PN component code lengths. Creating a composite PN code from individual component PN codes permits the acquisition of the first, which is usually the shortest, PN component code first. After lock-on employing code locked loops, the next, which is usually the next shortest, PN component code is acquired. Sequentially, each of the PN component codes may thus be acquired in order of increasing length until all PN component codes are acquired so as to reproduce the PN composite code.
Thus, there is a need to generate multiple sets of orthogonal pairs of PN codes. The present invention fulfills this need.
Using coherent signal receiving apparatus to receive data signals has several advantages. As a prime example, coherent reception of data improves or increases the ratio of the energy contained in the data bits to the energy of the background noise. This is similar to an improved signal to noise ratio. Also, coherent reception greatly improves the circuitry and structure necessary to receive the transmitted data signals. Coherent reception of data modulated onto a carrier implies that the absolute phase of the carrier signal is known so that the carrier signal can be tracked and removed at the receiver.
One prior art coherent receiving system employs a pilot signal which is not modulated by the data signal. The unmodulated pilot signal may be tracked and removed, and enables the receiving system to determine the absolute phase of the carrier signal. When the absolute phase of the carrier signal is known, the carrier signal may also be removed, leaving the demodulated data signal which may then be coherently detected. When this type of coherent modulation is employed at the transmitter, a portion of the total available power is consumed by the pilot signal and is so not available at the transmitter for data transmission nor at the receiver for detection.
U.S. Pat. No. 4,435,822 issued Mar. 6, 1984 for "Coherent Spread Spectrum Receiving Apparatus" by B. M. Spencer et al. explains that the pilot signal may be transmitted over a very short period of time to permit the receiving system to lock onto the transmitted pilot signal. The data signal is then transmitted without the pilot signal. In such an improved time-sharing system, the power available for data transmission is thereby enhanced. Also, in such a time-sharing system, less time is available for the transmission of data which effectively results in a slight degradation of the signal-to-noise ratio.
Both of the above-identified coherent receiving systems require special designed tracking circuitry to achieve the coherent reception of data.
It would therefore be desirable to provide a coherent spread spectrum receiving system that does not degrade the signal-to-noise ratio of the transmitted data signal and does not require any special design tracking circuitry to achieve coherent reception of data.